Bump gateway version to 0.8.7
Bump gateway version to 0.8.7

This commit bumps gateway version to 0.8.7 in docker-compose.yml and
also adds `scale_from_zero` environment variable to the gateway.

Fixes: #780

Signed-off-by: Vivek Singh <0309897457bdbaf8410ee5929728bb5df02fed57@yahoo.in>

rgee0/faas,alexellis/faas,alexellis/faas,openfaas/faas,rgee0/faas,alexellis/faas,alexellis/faas,alexellis/faas,rgee0/faas,rgee0/faas,alexellis/faas,alexellis/faas,rgee0/faas,rgee0/faas,rgee0/faas,openfaas/faas,alexellis/faas,openfaas/faas,alexellis/faas,rgee0/faas,rgee0/faas,rgee0/faas